In automation technology, especially in process automation technology, field devices are applied that serve for determining and monitoring process variables. Examples for such field devices are fill-level measuring devices, flow measuring devices, analytical measuring devices, pressure, and temperature, measuring devices, moisture, and conductivity, measuring devices, and density, and viscosity, measuring devices. The sensors of these field devices measuring device the corresponding process variables, e.g. fill level, flow, pH-value, substance concentration, pressure, temperature, moisture, the conductivity, density and viscosity.
The terminology ‘field devices’ includes, however, also actuators, e.g. valves or pumps, by which, for example, the flow of a liquid in a pipeline or the fill level in a container is changeable. A large number of such field devices are available from members of the firm, Endress+Hauser.
As a rule, field devices in modern automation technology plants are connected via communication networks, such as HART-multidrop, point to point connection, Profibus, Foundation Fieldbus, with a superordinated unit, which is referred to as control systems or control room. This superordinated unit serves for process control, process visualizing, process monitoring as well as for start-up and for servicing the field devices. For the operation of fieldbus systems, necessary supplemental components, which are directly connected to a fieldbus, and which are especially used for communication with the superordinated units, are likewise frequently referred to as field devices. These supplemental components include e.g. remote I/Os, gateways, linking devices or controllers.
The software portion of field devices is increasing increases steadily. The advantage of using microcontroller controlled field devices is that, via application-specific software programs, a variety of functionalities can be implemented in a field device; also program changes are relatively simple to perform. Standing in contrast to the high flexibility of the program controlled field device is a relatively low processing speed and therefore a correspondingly low measuring rate. This is a consequence of the sequential progression through the program.
In order to increase the processing speed, ASICs—Application Specific Integrated Circuits—are always applied when sensible in the field devices. Through their application-specific configuration, these chips can process data and signals substantially faster than a software program can. ASICs are excellently suited for computationally intensive applications.
Disadvantageous in the case of the application of ASICs is that the functionality of these chips is predetermined. In such case, subsequent change of the functionality is not directly possible. Furthermore, the use of ASICs is useful only in the case of relatively large lots, since the developmental effort and the associated costs are high.
In order to avoid this drawback of the fixedly predetermined functionality, a configurable field device is known from published International Application WO 03/098154, in which a reconfigurable logic chip in the form of a FPGA is provided. In the case of this known solution, during system start, the logic chip is configured with at least one microcontroller, also called an embedded controller. After the configuration is finished, the required software is loaded in the microcontroller. The required reconfigurable logic chip must make use of sufficient resources, namely logic, wiring and memory resources, in order to fulfill the desired functionalities. Logic chips with many resources require a lot of energy, which, in turn, from a functional point of view, makes their use in the process automation possible without limitation. The disadvantage of using logic chips with few resources and, thus, with less energy consumption is the considerable limitation in the functionality of the corresponding field device.
Depending on the application, the field devices must satisfy a variety of safety requirements. In order to satisfy the respective safety requirements, e.g. the SIL standard, ‘security integrity level’, the field devices must be designed with redundancy and/or diversity.
Redundancy means increased safety through double or multiple layouts of all relevant safety hardware and software-components. Diversity means that the hardware components, such as a microprocessor, which are located in the different measuring paths, come from different manufacturers and/or that they are of different types. In the case of software-components, the diversity requires that the software stored in the microprocessors originates from different sources, e.g. from different manufacturers or programmers. Through all these measures, it should be assured that a safety critical failure of the measuring device, as well as the occurrence of concurrent systematic errors in the providing of measured values, is excluded with a high degree of certainty. It is also known supplementally to design individual essential hardware, and software, components of the evaluating circuit redundantly and/or diversely. Through the redundant and diverse design of individual hardware, and software, components, the degree of safety can be still further increased.
An example of a safety-relevant application is fill-level monitoring in a tank, in which a flammable liquid is stored, or a liquid that is not flammable, but water endangering. This assures that the supply of liquid to the tank is immediately interrupted, as soon as a maximum allowable fill level is reached. This in turn implies that the measuring device very reliably detects the fill level and works faultlessly.